Barack Obama took the oath of office today as the 44th president of the United States and pledged to "begin again the work of remaking America."

Addressing a huge throng estimated at nearly 2 million people on the capital's Mall and millions of others watching on television, Obama somberly recognized the multiple crises now afflicting the nation at a time of war abroad and economic turmoil at home. But he sought to rally Americans to a "new era of responsibility" and the promise of a brighter future.

He cited a profound "sapping of confidence across our land -- a nagging fear that America's decline is inevitable, and that the next generation must lower its sights." He told Americans, "The challenges we face are real. They are serious, and they are many. They will not be met easily or in a short span of time."

"But know this, America -- they will be met," he declared to cheers from the crowd. "On this day, we gather because we have chosen hope over fear, unity of purpose over conflict and discord. On this day, we come to proclaim an end to the petty grievances and false promises, the recriminations and worn-out dogmas that for far too long have strangled our politics."

Obama continued: "Starting today, we must pick ourselves up, dust ourselves off, and begin again the work of remaking America."

Later, after a lunch at the Capitol in which Sen. Edward M. Kennedy (D-Mass.) suffered a seizure, Obama and the new first lady, Michelle Obama, rode in motorcade to the White House and twice stepped out of their heavily armored black limousine to walk parts of the 1.7-mile route and wave to cheering crowds.

As the motorcade reached the section of Pennsylvania Avenue that runs past the National Archives and the Navy Memorial, the Obamas got out and walked for about eight minutes. They subsequently took another walk of about six minutes, greeting well-wishers who stood behind barriers. The president wore a black overcoat, burgundy scarf and black gloves to ward off the late-afternoon chill, with the temperature hovering around 28 degrees, but the wind chill making it feel more like 16 degrees.

Police and troops lined the route, and Secret Service agents walked alongside the presidential limo behind a phalanx of police motorcycles with sidecars in "V" formation.

Obama, 47, began the final steps on his journey to the presidency this morning by attending a church service and paying a visit to White House for coffee.

The countdown to Obama's oath-taking came as Americans gathered in Washington in record numbers for the inauguration, braving midwinter cold and heavy security to witness an event -- the swearing-in of the nation's first African American president -- that for many marked a dramatic break with the past and the dawn of a new sense of possibility.

In his much-anticipated inaugural address, Obama sketched out a broad vision for the coming years in domestic policy, while urging rejection of the "false choice" in foreign policy between security and American ideals.

After an invocation by Pastor Rick Warren, a conservative who was included as a way to reach out to a segment of the population that did not support Obama, Joseph R. Biden Jr. took the oath as vice president.

Then, following a musical interlude, Obama stepped onto the stage and was sworn in, with a brief glitch in the oath-taking, by Chief Justice John G. Roberts Jr. at 12:05 p.m. Holding the Bible was Obama's wife, Michelle Obama.

"What is required of us now is a new era of responsibility -- a recognition, on the part of every American, that we have duties to ourselves, our nation and the world, duties that we do not grudgingly accept but rather seize gladly," Obama said.

Only once in his 20-minute speech did he refer directly to America's segregated past, and to his own race as the son of a black father from Kenya and a white mother from Kansas.

"This is the meaning of our liberty and our creed -- why men and women and children of every race and every faith can join in celebration across this magnificent Mall, and why a man whose father less than 60 years ago might not have been served at a local restaurant can now stand before you to take a most sacred oath," Obama said, drawing the loudest cheers of his speech.

"So let us mark this day with remembrance, of who we are and how far we have traveled."

The new president pledged to create new jobs, build up the nation's infrastructure and "restore science to its rightful place."

He said the United States would "begin to responsibly leave Iraq to its people, and forge a hard-earned peace in Afghanistan." He pledged to work with other nations to fight the dangers of nuclear arms and the threat of global warming.

But he also warned: "We will not apologize for our way of life, nor will we waver in its defense, and for those who seek to advance their aims by inducing terror and slaughtering innocents, we say to you now that our spirit is stronger and cannot be broken; you cannot outlast us, and we will defeat you."

Obama extended a hand to the Muslim world, while also cautioning that leaders who sow conflict or blame their ills on the West will be judged by their people "on what you can build, not what you destroy." And to those who "cling to power" through corruption, deceit and repression, he said: "Know that you are on the wrong side of history; but that we will extend a hand if you are willing to unclench your fist."

Addressing people around the world, he drew more cheers when he said, "America is a friend of each nation and every man, woman and child who seeks a future of peace and dignity, and we are ready to lead once more."

During and after the speech, many in the audience could barely contain their emotions.

"I knew I was going to cry because I've been crying since November," said Judith Clark, 47, who made sure to bring tissues to spare on her trip to Washington from her home in Atlanta. "I just can't believe we made it to the promised land."

"We have to work together to help our president as African Americans," said Margia Blankenship, 58, a hotel developer from Houston. "Really, now is the time for all Americans to help our president."

At the Lincoln Memorial, Dwyan Turner West, 36, a tech manager for the U.S. Army, stepped to the spot where Martin Luther King Jr. delivered his "I Have a Dream" speech in 1963. He bowed his head, clasped his hands and then saluted, his angular face taught, his eyes blinking. At the end of the inaugural address, he raised his fists in the air, and a white stranger leaned over and said, "Welcome to the new America."

Turner West shook his head and smiled. "We did it," he said. "We finally did it. It's here."

After the ceremony, Barack and Michelle Obama bade farewell to President Bush and his wife, Laura Bush, on the eastern steps of the Capitol. The couples embraced before the Bushes boarded a Marine helicopter en route to Andrews Air Force Base and a flight to their home state of Texas.

The Obamas then repaired to the Capitol's Statuary Hall for a traditional luncheon before this afternoon's inaugural parade.

During the lunch, Kennedy fell ill and was rushed away for treatment. The 76-year-old senator, who was diagnosed last year with brain cancer, apparently suffered a violent seizure and was taken to a hospital in an ambulance, officials said.

Obama paid tribute to Kennedy at the end of the luncheon. "I would be lying to you if I did not say right now a part of me is with him," the new president said.

In brief remarks following an exchange of toasts, Obama told the gathering, "What's happening today is not about me; it is about the American people. They understand we have arrived at a time of great challenge for our nation."

Americans "have come together across races and regions and stations; now we have to do the same," Obama said.

Shortly after 3:15 p.m., the Obamas and Bidens were escorted down the eastern steps of the Capitol to mark the start of the inaugural parade. Military bands, including a fife and drum corps in red tunics and tricorn hats, marched past before the presidential motorcade set off on its trip to the White House.

After the president arrived at an enclosed reviewing stand in front of the White House, participants in the parade, including marching bands and military formations, began to flow past.

This morning, making their first public appearance of the day, the Obamas emerged from their temporary residence at Blair House at 8:47 a.m. Eastern time and stepped into their limousine for the two-minute drive to St. John's Episcopal Church, where Biden and his wife, Jill Biden, awaited them. The Obamas were running about 15 minutes behind schedule.

"Through you, may God bless America in a way that we are a blessing to the whole world," one of the first speakers, Pastor Joel Hunter, told Obama.

Bishop T.D. Jakes, a senior pastor from Houston, used Scripture to offer the incoming president four lessons for his administration. "In time of crisis, good men must stand up," Jakes said. "God always sends the best men into the worst times." He also told the worshipers, "This is not a time for politeness or correctness; this is a time for people to confront issues and bring about change. . . . You cannot enjoy the light without enduring the heat."

Looking directly at Obama, Jakes said, "The problems are mighty and the solutions are not simple, and everywhere you turn there will be a critic waiting to attack every decision that you make. But you are all fired up, sir, and you are ready to go. And this nation goes with you. God goes with you."

"I say to you as my son who is here today, my 14-year-old son -- he probably would not quote Scripture. He probably would use Star Trek instead. And so I say, 'May the force be with you.' "

After attending the traditional pre-inauguration church service for about an hour, the new first couple and the Bidens were driven across Pennsylvania Avenue to the White House for a meeting over coffee with President Bush, Vice President Cheney and congressional leaders. Upon alighting at the North Portico, the Obamas were met by the Bushes. Michelle Obama, wearing a sparkling gold sheath dress with matching long coat, handed a gift-wrapped box to Laura Bush, and the couples chatted for a moment before entering the White House on a red carpet between saluting guards. The first lady's office said the gift was a pen and a journal for Laura Bush's memoir writing.

Cheney, who is using a wheelchair after injuring his back moving boxes, did not appear at the entrance to greet his successor.

About 50 minutes later, Obama rode with Bush to the Capitol for the swearing-in ceremony, Biden traveled in a separate limousine with Cheney, and the incoming and outgoing first ladies were driven in a third.

At the Capitol, they joined a stream of dignitaries including former presidents and vice presidents, members of the Supreme Court, lawmakers and Cabinet officers of the Bush and Obama administrations.

Beginning well before dawn, people streamed onto the Mall between the Capitol and the Lincoln Memorial to claim their places for the historic occasion.

They came from the District, the surrounding region and from across the country to watch Obama place his right hand on Abraham Lincoln's Bible and swear to uphold the Constitution -- 148 years after his historical hero and fellow Illinois political leader embarked on a mission to preserve the Union and end slavery.

"All my ancestors who gave their blood, sweat and tears made this day possible," said District resident Janice Leek, 54, as she huddled against the cold on the Mall with her daughter and a friend from Detroit. "This is an amazing day."

Today's transfer of power wrote an end to Bush's eight-year tenure, a tumultuous period that has bequeathed the incoming administration two wars, record budget deficits, unprecedented national debt and the worst economic crisis since the Great Depression.

Obama took the oath of office under the tightest security for an official event in the city's history, with a large swath of downtown Washington closed to vehicular traffic. About 28,000 law enforcement and military personnel were deployed in and around Washington to provide protection for the oath-taking and the massive throng gathered on the Mall and along the inaugural parade route to see history in the making.

The security force was more than 50 percent larger than the contingent assembled four years ago for Bush's second inauguration.

The force included about 10,000 National Guardsmen from two dozen states and the District, at least 2,000 active-duty military members, 8,000 police officers, roughly 4,000 Secret Service members and thousands of other federal agents and law enforcement officers from the U.S. Park Police, the Capitol Police, the FBI and other agencies.

Of the approximately 8,000 regular police officers, about half were from the District's department and half from police departments across the Washington region and the country. The police on loan from other departments were essentially deputized by the D.C. police, temporarily doubling the ranks of the city's police force.

In addition, about 5,000 U.S. military members were participating in ceremonial roles, inauguration organizers said.

Despite the troubles confronting him at home and abroad, Obama assumes the presidency amid high public expectations and widespread optimism that his programs will work. According to a new Washington Post-ABC News poll, 80 percent of Americans approve of the way Obama has handled the presidential transition, 72 percent are confident that his economic programs will improve the economy and 79 percent have a favorable impression of him. By comparison, 62 percent had a favorable impression of Bush when he took office in January 2001, and 33 percent now approve of his performance as president.

Obama's history-making quest began in early 2007 when he announced his candidacy for the Democratic presidential nomination, an audacious move for a freshman U.S. senator who had only recently stepped onto the national stage from the Illinois state Senate. But his ambitions began to take shape long before he entered politics. Shortly before his 1992 marriage to Michelle Robinson, Obama told his future brother-in-law that he planned one day to seek national office, possibly even the presidency.

His main rival in the Democratic primaries, Hillary Rodham Clinton, and his opponent in the presidential election, Sen. John McCain (R-Ariz.), charged that Obama was too inexperienced for the nation's highest office. But Obama projected himself as a level-headed decision-maker and relied on soaring oratory to inspire millions to believe in him, rallying voters with the slogan "Yes, we can!"

He captured 53 percent of the popular vote in the Nov. 4 election, easily winning the presidency with 365 electoral votes to 173 for McCain.

Since then, he has reached out to former rivals and Republican leaders, appointing Clinton as his secretary of state and pledging -- at a dinner for McCain last night -- to "usher in a new season of cooperation" and bipartisanship.